1. Field of the Invention
This invention relates generally to air displacement pipettes, and is concerned in particular with an improvement in pipette tips and the manner in which they are releasably retained on and ejected from the tubular mounting shafts of the pipettes.
2. Description of the Prior Art
It is known to detachably retain a pipette tip on the tubular mounting shaft of an air displacement pipette. The pipette is equipped with a manually operable ejection mechanism for disengaging and releasing the thus retained pipette tip once it has served its purpose. Retention is commonly achieved by effecting a friction fit between coacting surfaces on the pipette tip and the mounting shaft.
This leads to certain difficulties in that users are often uncertain as to the level of force required to achieve a secure friction fit. An inadequate force can result in the pipette tip becoming prematurely dislodged, whereas an excessive force can result in the pipette tip being jammed in place, which in turn disadvantageously increases the force that must be exerted by the manually operable ejection mechanism when dislodging the pipette tip from its retained position. These problems are exacerbated in multi channel pipettes.
It is also known to provide the cylindrical walls defining the upper ends of the pipette tips with interiorly projecting circular ribs or ridges designed to coact in snap engagement with mating surfaces on the tubular mounting shafts of the pipettes.
However, this also leads to certain difficulties in that in order to achieve a snap engagement, the upper walls of the pipette tips must be radially expanded, which in turn requires the user to exert unacceptably high forces when axially inserting the tubular mounting shafts into the pipette tips. Comparable forces are required to disengage the tips from the mounting shafts. Moreover, slight dimensional variations can have a significant impact, e.g., by either additionally increasing the forces required to engage and release the pipette tips if their internal wall diameters are too small, or resulting in unacceptably loose connections if their internal wall diameters are too large.